Various modular prostheses have heretofore been designed to replace one or both ball and socket hip joints. Generally, a body component is connected to a stem component, which is embedded in the intramedullary canal of the proximal femur for hip reconstruction. An artificial ball is attached to the body component and received in a socket portion, which is attached to the hip bone. The recognized advantage of modular components lies in the ability to custom fit the prosthesis by having available a multitude of different sized and shaped body and stem components, all of which are attachable to one another. One such example includes the modular body revision series manufactured by Biomet, Inc. and sold under the trade name Mallory-Head.RTM. Hip Program. In that product line, three different stem lengths are available in five different diameters, and three differently shaped body components are each available in five different sizes. Thus, in that product line physicians have the ability to assemble two hundred and twenty-five different prostheses utilizing different combinations of the available components.
In addition to having the ability to assemble literally hundreds of different prostheses from available components, further variations are available by the fact that the modular components can be assembled to one another in any angular orientation. While this angular orientation is generally not important in those cases where the stem portion is straight and does not include a coronal slot, it is critically important in those cases where the stem includes a coronal slot and/or includes a bowed portion to match the bow in the bone being replaced. The coronal slot orientation reduces bending stress along the coronal plane, and therefore its orientation is of critical importance to the long-term success of an implanted prosthesis in preventing fracture after implantation.
In many modular component systems, provisional components are positioned at the implantation site in order to precisely size and determine the desired angular orientation for the actual prosthesis. While the use of provisional components allows the physician to accurately size the modular prosthesis, accurately transferring the angular orientation of the components from a provisional implant to the actual modular prosthesis has been somewhat more problematic. One known method for transferring the angular orientation of the provisional implant to the actual prosthesis is described in U.S. Pat. No. 5,135,529 to Paxson, et al. In this patent, angular markings on the implants themselves are aligned to correspond to a vertical line etched on the stem component. However, because the angular markings are rather crude and because they are located a relatively short distance from the axis of rotation, the procedure described in Paxson, et al. can still result in an angular deviation from a desired orientation as much as plus or minus 10.degree.. Other known systems simply rely only upon the physician's estimate, and consequently result in even cruder settings of the angular orientation between the modular components. Thus, while modular components have the ability to provide a precise size fitting, there remains significant room for improvement in setting the angular orientation of the components relative to one another in order to provide an even better precisely matched fit for the implantation of a prosthesis in an individual patient.